How to build (digital) safety for whistleblowers?

In 2013, The Publeaks Foundation and a large number of Dutch media outlets launched Publeaks.nl, a website for people to leak documents to the media securely and anonymously. Since then, a number projects have been launched based on the Publeaks Method: AfriLeaks, WildLeaks, Sourcesure and MexicoLeaks. How do these platforms work? What methods, software and hardware is used? What digital skills are needed from the journalists?